In the case of an extreme ultraviolet light source at 13.5 nm produced by a lithium plasma, it is difficult to design the electrode for least heat flux because the EUV light can carry even more heat than the plasma particles, and its propagation is not hindered by protection via for example an applied magnetic field. This combined flux of particles and EUV light can raise the electrode surface temperature transiently, raising the danger that the melting point of the electrode material could be approached, followed by flow of surface material and deformation of the electrode shape.
The general principle of surface protection via liquid metal from concentrated electric discharge heat is long established, having been the subject of Swiss Patent 301203, titled “Ignitron”, awarded to Westinghouse in 1954. In that work the liquid metal, mercury, was delivered to the surface area containing the discharge location via porosity of the underlying substrate, which typically was a porous matrix of sintered tungsten or molybdenum. That approach allowed the ignitron to operate on the first pulse from cold, independent of device orientation, and protected the substrate from plasma erosion during a long operating life.
Subsequently, in U.S. Pat. No. 7,518,300 awarded to Philips, a discharge between electrodes, at least one of which is constructed from a matrix material or a carrier material, is claimed to reduce electrode erosion by providing charge carriers via evaporation and ionization of a sacrificial substrate material disposed within or upon the matrix or carrier material. The evaporated material may be partially ionized by a laser or other means to provide carriers for the discharge, thereby localizing the discharge and at the same time protecting the underlying carrier material.